Modulating apparatus for telegraph signals



MODULATING APPARATUS FOR TELEGRAPH SIGNALS Filed July 29, 1919 INVENTOR [@dlymwazkai BY W;

ATTORNEY Patented Dec. 8, 1925.

UNITED STATES PATENT OFFICE.

LLOYD ESPENSCHIED, OE HOLLIS, NEW YORK, ASBIGNOR-TO AHERICAN TELEPHONE AND TELEGRAPH COMPANY, A CORPORATION 01 NEW YORK.

IMIODULATING APPARATUS FOR TELEGRAPH SIGNALS.

Application filed July 29, 1919. Serial a... 314,075.

To all whom it may concern:

Be it known that I, LLOYD residing at Hollis, in the county of Queens and State of New York, have invented certain Impr-o-vementsin Modulating Apparatus for Telegraph Signals, of which the fol lowing is a specification.

This invention relates to multiplex transmission and. more particularly to systems either wire or wireless in which. carrier currents are employed for the transmission of signals.

It has dinary low frequency transmission line, such as a telephone or a telegraph line, by superimposing thereon a plurality of carrier currents of different frequencies, said carrier currents being used for the selective transmission of telephone or telegraph signals \Vhere the carrier current channels in such a system areto be employed for the transmission of telegraph signals, or where telegraph signals are transmitted by radio method, it has been customary to transmit the signals by making and breaking the carrier circuit in such a manner as to interrupt the carrier oscillations, either high frequency or low frequency, which are transmitted over a given channel.

This method of operation has certainobjections. For instance, the making and breaking of the circuit results in impressing substantially a square topped telegraph wave upon the carrier circuit. This gives rise to oscillations known as transients in the selecthey may extend so 'far in the frequency spectrum as to overlap the bands of the adjacent channels and thereby set up interfering crosstalk currents in the selective circuits of these adjacent channels.

Another objection to the system heretofore used arises from the fact that it-is frequently desirable from an operating standpoint to Esrnnscmnp,

been proposed to multiplex an orlocate the control relay which makes and breaks the carrier circuit some little distance from the carrier apparatus proper, the long leads extending from the control relay to the carrier apparatus comprising an appreciable capacity tending .to short circuit the relay contacts and permitting current to escape the control relay. Furthermore, the length of these leads gives an opportunity for-high frequency crosstalk in the station wlrmg.

In order to overcome these objections it is proposed, in accordance with this invention, to impress the telegraph wave upon carrier alternations by modulation instead of mechanical interruption. Preferably, the modulation will be accomplished by means i of vacuum tubes, although other modulating arrangements may be employed. In the case of the vacuum tube modulator, while the telegraph .wave maybe impressed upon either the input or out ut circuit of the modulator, it is herein ilustrated as being applied to the input grid filament circuit.

A filter is provided in the modulating circuit ing wave causes the tube to operatearound' the curved portion of its characteristic curve thereby changing the amplification of the carrier current and aflecting the modulation thereof.

The invention may now be more fully understood by reference to the following detailed description, when read in connection with the accompanying drawings, the figure of which constitutes a circuit diagram illustrating certain embodiments of the invention.

Referring to the drawing, ML designates an ordinary low frequency transmission line, such as a telephone circuit,.said line being provided with the usual composite set Cw, whereby the'circuit may be used for the simultaneous transmission of telephone signals and low frequencyMorse signals. The main line MD is provided with a branch LT leading througlra low pass filter LF connections over to a jack llF are of the general type disclosed in- United States patents to Geor .bell, Nos. 1,227,113 and 1,22

e A. Camp- ,114, dated May 22, 1917, and as set forth in said patents may be designed to transmit any desired range of frequencies. The filterLF is so proportioned as to transmit frequencies running from zero to the upper limit of frequencies employed in ordinary telephonic transmission, which may be in the neighborhood of 2200 cycles. High frequency carrier oscillations are substantially suppressed by the filter and prevented from being transniitted to the terminal branch LT. The filter HF, on the other hand, is so proportioned as to suppress telephonic frequencies while transmitting frequencies within the carrier range. The convenient cut off point for the filter- HF is 3000 cycles, although this is to be understood as purely illustrative and not as a limitation inherent in the arrangement.

The carrier branch GB is associated with a common transmitting circuit TL and a common receiving circuit RL through a balanced transformer arrangement 10. In order to render the circuits TL and BL conjugate to each other, so that electrical variations in the one circuit will be without effeet upon the other, a balance circuit MN is provided, including a filter HF or equivalent apparatus for balancing the filter HF, and branches X and Y. The branch X includes a filter LF' or equivalent structure for balancing the filter LF in the terminal line LT, and a resistance TN for balancing the terminal line which may be associated with the jack J. The branch Y includes a network consisting of a capacity 20 and resistance 21 which may be adjusted to balaucethe main line circuit ML.

The common transmitting circuit TL is associated with a plurality of transmitting channels through filtering arrangements, such as T 1 T F, T 13, T,,F, and so forth, which are preferably in the form of sharply tuned circuits. Each of the channels has assigned to it a definite carrier frequency to which the filter elements are tuned, the assigned frequencies being indicated as 7-250 cycles for the first channel, 7750 cycles for the second channel, 8250 cycles for the thirdchannel and 8750 cycles for the fourth channel. These frequencies are indicated merely for purpose of illustration, it be iug understood that any desired frequencies may be assigned to the various channels so long as the frequencies are sufiiciently separated to provide the desirable degree of selectivity.

The carrier apparatus of the first channel includes an oscillating tube T V, of well known form, having its output circuit coupled to its input circuit in a well known manner so that the tube oscillates at any desired frequency. The circutts of the tube are so adjusted that its oscillates at the frequency assigned to its particular channel,

which in the case illustrated is 7250 cycles.

A modulating tube T,V is also provided for the purpose of amplifying and modulating the oscillation supplied by the oscillator T V. The input circuit of the modulating tube T,V' .is associated with the output circuit of the tube T,V so that the oscillations from the latter tube will be impressed upon the former. In order to impress the telegraph signals upon the carrier oscillations in such amanner as to modulate the carrier currents, a modulating circuit M 0 is connected across the grid filament circuit of the modulatingtube T,V as indicated. The modulating circuit M,C includes a battery 13, which may 'have its negative terminal connected to the grid of the tube T,V"under the control of the Morse relay M,R thereby varying the grid potential of the tube T,V in'accordance with the telegraph signal. A filter F is included in the modulating circuit M,() for 'the purpose of rounding off the Morse impulses supplied by the battery B, and for preventing high frequency oscillations from reacting in a portion of the modulating circuit M,C extending to the Morse relay M,R. This relay may be located at a point distant from the modulating apparatus and may even be located at a separate station. The filter F is preferably located near the modulating apparatus so that high frequency oscillations will not be transmitted over the portion of the circuit M,C extending to the Morse relay, thereby substantially preventing transmission loss in the carrier circuit due to the shunt capacity of the modulating circuit and preventing also crosstalk in the station wirmg.

The carrier apparatus for the other channels. may be similar to that already described. In order, however, to illustrate another form of modulating arrangement, a modified form of apparatus is illustrated in connection with the fourth carrier channel. In the modified arrangement the oscillating tube T,V functions both as an oscillator and a modulator while the tube T,V functions merely as an amplifier. In order to secure this result the modulating circuit M 0 is connected'to the grid filament circuit of the oscillating tube T V as illustrated and includes the filter F for the same purpose as the filter I in the first form. The Morse. relay M 1} controls the application of poten tial from the battery I}, to the grid of th oscillating tube T,V so that the amplitude of the oscillations enerated by the tube may be varied in accordance with the Morse signals.

The common receiving circuit BL is associated with the individual receiving channels through filters, such as, R 1 R 1, B l 12 1*, and so forth, which are preferably, in the form of sharply tuned circuits, tuned to the particular frequency assigned to the corresponding receiving channel.

The apparatus associated with the first receiving channel only is illustrated and comrises ampli ing and detecting tubes 3 V, V and R These tubes are provided with circuits of the usual type for amplifying and detecting and need not .beofurther v described. A receiving relay R,R is included in the output circuit of the last tube in the series and responds to the detected Morse signals impressed upon the carrier frequency assigned to its particular channel. The car'- rier apparatus for the other channels may be similar to that above described.

The arrangements hereinbetore discussed constitute the apparatus at one terminal station of the main line ML. It will be understood that similar arrangements will be provided at the distant terminal station. It will also be noted that the carrier fre uencies assigned to the several receiving 0 annels which, for purposes of illustration, are indicatedv as being 7500, 8000, 8500 and 9000 cycles, respectlvely, are intermediate between the severalfrequencies of the corresponding transmitting channels. Thls staggered ar- 1 rangement of the transmitting and receiving frequencies serves to supplement by a trequeney separation the separation due to the balanced relation between corresponding transmitting and receiving channels. In actual practlce it is impractical to secure accurate balance between the transmission line and the artificial balancing arrangement so that by transmitting and receiving at difi'erent frequencies the oscillations transmitted from, say the first channel and reacting 1n the common receiving circuit RL due to the slight unbalance existing, will be prevented from being transmitted to any of the individual receiving channels due to the fact that it is filtered out by the tuned circuits. It will be understood, of course, that the frequencies assigned tothe transmitting channels at the distant station will be the same gether with oscillations from the other transmitting channels, are impressed upon the common transmitting circuit TL. The Morse relay M,R operating in response to Morse signals opens and closes circuit M,C and applies impulses of direct current across the grid filament circuit of the modulating tube T 13; These impulses vary the potential on the grid of the modulating tube and consequently vary the amplitude of-the oscillations transmitted through the filter T F. The direct current impulses in the modulating circuit M 0 are not square topped waves owing to the fact that they are rounded oil by the action of the filter F Consequently, the transients which would tend to be produced by the sudden reaction of a square topped wave upon a tuned circuit are considerably reduced and the corresponding cross-talk effect between adjacent tuned circuits is to a large extent eliminated.

The high frequencyoscillations impressed upon the common transmission circuit TL from the various transmitting channels are transmitted through the filter HF and over the common branch CB to the main line ML for transmission to the distant station Where they are separated in the proper receiving channels and detected.

The receiving operation may be understood from a consideration of the action at the station illustrated with respect to transmission from the distant station. Assuming that oscillations corresponding to the several carrier frequencies are modulated by Morse signals at the distant station and transmitted over the line ML from the distant station, the several modulated frequencies will pass into the carrier branch CB and through the filter HF. being prevented from flowing to the terminal line LT by means of the low pass filter LF. quencies after passing through the filter HF are transmitted through the balanced transformer 10 to'the common receiving circuit RL and are separated by means of the filters R F, R F, and so forth, into the corresponding receiving channels. Modulated oscillationshaving normal carrier frequencies of 7500 cycles pass through a filter R,F and are amplified and detected by the tubes RV. R,V and R V" in a well known manner. The low frequency detected currents corresponding to the modulating Morse signals operate the receiving relay RJt in a well known manner.

The operation of the modified form of modulating apparatus illustrated in conuection with the fourth transmitting channel is similar to that already described. The tube T V oscillates at the "frequency corresponding to the channel and the oscillations supplied are amplified by the amplifier T .,V and transmitted through the filter T 1 to the common transmitting circuit TL. The

The several freti l Morse l relay ME in response to Morse impulsesl applies direct current impulses to the grid circuit of the oscillating tube T V, thereby varying the potential of the grid circuit to correspondingly vary the amplitude of the oscillations generated. The filter 1*, curbs or rounds off the Morse waves applied to the modulator to reduce the transients resulting from square topped waves and prevent high frequency oscillations from reacting in the modulating circuit M,C.

By means of the arrangement above disclosed the high frequency current can be kept out of the local control circuit by means of the filter included in such circuit and the transmitting relay can be located at any point in the operating room or at the dis tant end of a line extending to another station. The low frequency waves impressed upon the carrier oscillations can also be curbed toany desired extent thereby relieving the high frequency selective circuits of some or all of this duty With the consequent in'iprovement in their selective action.

il hile the invention has been illustrated as embodied in certain specific forms, it will be obvious that the general principles herein disclosed may be embodied in many other organizations Widely different from those illustrated without departing from the spirit of the invention as defined in the following claims.

What is claimed is: p

1. In a transmission system, a modulator, a source of alternating carrier current, conncctions for impressing carrier currents from said. source upon said modulator, a controlling circuit: for supplying telegraph impulses to said modulator, and-a frequency selective filter in said controlling circuit, said filter having a substantially sharp cutoil and being so designed as to transmit uniformly and Without substantial attenuation frequencies below the cut-offpoint while substantially suppressing all frequencies above. the cut-off point, and the cut-0d point beinglocated well below. the carrier frequency but sufficiently high to transmit the essential frequencies of the telegraph impulses thereby preventing the carrier current from reacting in said controlling circuit.

ln a transmission system, a modulator, a source of alternating carrier current, connections for impressing carrier current from said source upon said modulator, a controlling circuit for supplying telegraph impulses to said modulator. and a frequency selective filter in said controlling circuit. said filter having a substantially sharp cut-oil and be' in; so designed as to transmit uniformly and with substantially negligible attenuation all frequencies below the cut-off point While substantially suppressing all frequencies above said cut-off point, the cut-off. point being so set as to eliminate the higher harmonics of the dot telegraph frequency thereby rounding oil the telegraph impulses and decreasing the transient oscillations due to sudden changes in the telegraph impulses.

3. In a transmission system, a vacuum tube modulator including a filament, grid and plate, an output circuit connected to the filament and plate and an input circuitconnected to the grid and filament, a source of alternating carrier current, connections for supplying carrier currents from said source to the grid filament circuit, a controlling circuit for supplying telegraph impulses to said grid filament circuit, and a frequency selective filter in said controlling circuit, said liltcr having a substantially sharp cutoll' and being so designed as to transmit uniformly and with substantially negligible attenuation all frequencies below the cut-oft point while substantially suppressing frequencies above the cut-oft point, the cut-off point of the filter being set well below the carrier frequency but sufiicient-ly high to freely transmit the essential frequencies of the telegraph impulses thereby preventing the carrier currents from reacting in said controlling circuit.

4. in a transmission system, a vacuum tube modulator including a filament, grid and plate, an output circuit connected to the filament and plate and an input circuit conncctcd to the grid and filament, a source of alternating carrier current, connections for supplying carrier current from said source to the grid filament circuit, a controlling circuit for supplying telegraph impulses to said grid filau'icntcircuit, and a frequency selective filter in said controlling circuit, said filter having a substantially sharp cutoff and being so designed as to transmit uniformly and with substantially negligible attenuation all frequencies below the cut-oil point while substantially suppressing all frequencies above the cut-off pointpthe cutoff point being so set: as to suppress the higher harmonics of the fundamental dot telegraph frequency thereby curbing said telegraph impulses and decreasing the t ansient oscillations due to sudden changes in the telegraph impulses.

5. ln multiplex' carrier telegraphy, the method of reducing interference between neighboring high frequency channels which comprises preventing the transmission of the unessential side frequencies of the high frequency wave of each channel before impressing the bands upon the channel selecting circuits, and then transmitting the bands thus narrowed through channel selecting circuits.

6. in high frequency multiplex telegraphy in which high frequency waves are varied in accordance with low' frequency signaling impulses, the method of reducing iii-- terference' between neighboring high freill) qucnc jy channels which comprises reducing the abruptness of the signal variations in the modulated high frequency waves.

7. In high frequency multiplex telegraphy in which high frequency waves are varied in accordance with low frequency signaling impulses, the method of reducing interference between neighboring high frequency channels which comprises reducing the abruptness of the variations in the high frequency waves by modifying the wave form of the low frequency signaling impulses.

8. In high frequency telegraphy in which a high fre uency wave is controlled in accordance with low frequency signaling impulses, th-emethod of eliminating the unes sential side frequencies accompanying a transmitted high frequency wave which comprises selectively attenuating the low frequency signaling impulses used to control said high frequency wave.

S). The method of high frequency telegra h transmission which comprises producing high frequency waves and low f-rcquency signaling impulses, attenuating selectively said low frequency signaling impulses to reduce their sharpness, and controlling said high frequency wave in accordance with the attenuated signaling impulses.

10. The method of producing a signalcontrolled high fre uencywave for telegraphy, unaccompanied by unessential side frequencies, which comprises producing a high frequency wave, producingtelegraph signal impulses, selectively attenuating said impulses to reduce their sharpness, and controlling said high frequency Wave in accordance with said attenuated signal impulses.

11. A signal transmitting system comprising a source of high frequency carrier waves,

a source of substantially abrupt signaling impulses, a circuit for transmitting said.

impulses, said circuit including a selective filter for reducing the sharpness of said impulses, the said filter having a substantially SllfiII) Cllt-Ofi and bein so designed as to transmit uniformly and with negligible attenuation all frequencies'b'elow the cut-off point while substantially suppressing all frequencies above the-cut-otf point, the cut-v off point being so set as to eliminate the higher harmonics of the fundamental dot frequency, and means for. controlling said high frequency wave in accordance with the signaling impulses as modified after transnnssion through the filter.

12. In a high frequenc signaling system, means for producing -hig frequency carrier waves controlled in accordance with low frequency signaling impulses, and means for eliminating the unessential side frequencies of said high frequency ,waves, said means comprising a selective filter through which low frequency impulses are transmitted, said filter having a substantially sharp cutoff and being so designed as to transmit uniformly and with substantially, negligible attenuation all frequencies below the cutoff point while substantially suppressing frequencies above the cut-oft point, the cutoff point being so set as to suppress the higher harmonics of the fundamental dot signaling impulses.

13. In a h gh frequency signaling system, means for producing a high frequency carricr vwave controlled in accordance with low frequency signaling impulses, and inc-ans for eliminating the unessential side frequencies of said high frequency wave, said means. comprising a selective filter through which the low frequency impulses are transmitted, said filter having a substantially sharp cut-off and being so designed as to transmit uniformly and with substantially negligible attenuation all frequencies below the cut-off point while substantially suppressing frequencies above the cut-off point, the cut-o point being so set as to eliminate the higher harmonics of the fundamental dot signaling impulses thereby reducing the sharpness of said impulses.

14. In ahigh-frequency signaling system, a source of high frequency carrier waves, means for transmitting. said waves, a circuit for controlling said waves in accordance with low fre uency signaling impulses, and a low pass ii ter in said circuit for transmitting said impulses, said filter having a substantially sharp cut-off and being so designed as to transmit uniformly and with substantially negligible attenuation all frequencies below the cut-off point while substantially suppressing all frequencies above the cut-off point, the cut-off point of the filter being so set as to eliminate the higher harmonics of the fundamental dot signaling impulses thereby reducing the abruptness of saidimpulses.

15. In a high, frequency signaling system, a source of high frequency carrier waves,

a device for acting upon said waves to control their transmission, a circuit for acting upon said device in accordance with signaling impulses in said circuit, and a low pass filter in said circuit for transmitting said signaling impulses, said filter having a substantially sharp cut-off and being so desi ned as to transmit uniformly and with su lostantially negligible attenuation all frequencies below the cut-off point while substantially suppressing all frequencies above the cut-off point, the cut-off point being ,so set as to eliminate the higher harmonics of the fundamental dot signaling impulses thereby rounding off said slgnaling impulses before acting upon said controlling device.

16. In a'high frequency telegraph system, a line, sources of high frequency waves for said line, means for varying the ener r of each high frequency were in accor ance with signal impulses to he'transmitted, and means for reducing interference between said different high frequency waves com prising filters for rcducingthe sharpness of the signal impulses used to vary the several high frequency waves.

17. In a high frequency signaling system, a source of high frequency carrier waves, a repeater for reproducing said waves for transmission, connections hetween said source and said re eater, a circuit for controlling the repro notion of said waves by said repeater, a source of signaling impulses in said circuit, and a frequency selective filter in said circuit, said filter having a substantially sharp cut-oil and loeing so de si ned as to transmit uniformly and with siilistaiitially negligible attenuation all frequencies below the cut-off point while substantially suppressing all frequenciesabove the cut-off point, the cut-off point being so set as to eliminate the higher harmonics of the fundamental dotsignaling im ulses thereby reducing the'ahruptness of said impulses.

18., In a high frequency telegraph system, a source of high frequency waves, an amplifier for said waves, connections between said source and said amplifier, a circuit for controlling said amplifier in accordance with telegraphic signals to transmit said waves and a low pass filter in said circuit for reducing the abruptness of said signals.

19. in a high frequency telegraph system, a generator of sustained high frequency waves, a circuit for producin telegraphic signals comprising a line inclu mg a source of direct current, and a key, se ective attenuating means in said circuit for reducing the abruptness of the makes and breaks by said key, a current controlling element, connections between said generator and said current controlling element, and

means for im 'ircssing said high frequency waves and the attenuated telegraphic sig: nals upon said element to control transmission of the high frequency wave in accordance with the impressed signals.

20. In a high frequency telegraph system, a transu'iittin amplifier comprising a suitably mountet cathode, anode, and a rid impedance controlling element, a grid atfilter in said circuit. said filter having a sub-- stantially sharp cut-oft and being so designed as to transmit uniformly and with substantially negligible attenuation all frequencics below the, cut-off point while suhstantially suppressing all frequencies above the cut-oil point, the cut-off point of said filter being so set as to eliminate the higher harmonics of the fundamental dot telegraph impulses thereby reducing the sharpness of the telegraphic impulses in said circuit.

In testimony whereof, I have signed my name to this specification this 28th day of July, 1919.

LLOYD ESPENSCHIED. 

